template_english - rubberjournal.rubber.or.kr › data › template(en).docx · web viewand...
TRANSCRIPT
Self-Adhesion Behavior of Uncured Brominated Isobutyl Isoprene Rubber
Nanoclay Composites
Bismark Mensah*, Sungjin Kim*, Dae Hak Lee*, Han Gil Kim*, Jong Gab Oh**,
and Changwoon Nah*,**,†
* Department of Polymer-Nano Science and Technology, Chonbuk National University, Jeonju
561-756, South Korea
** Department of Design and Manufacturing, Graduate School, Chonbuk National University,
Jeonju 561-756, South Korea
_____________________________
† Correspondence should be addressed.
E-mail: [email protected]
ABSTRACT: The effect of nanoclay (Cloisite 20A) on the self-adhesion behavior of uncured
brominated isobutylene isoprene rubber (BIIR) has been studied. The dispersion state of
nanoclay into the rubber matrix was examined by SEM, TEM and XRD analysis. The thermal
degradation behavior of the filled and unfilled samples was examined by TGA and improvement
in the thermal stability of the nanocomposites occurred based on the weight loss (%)
measurements. Also, addition of nanoclay enhanced the cohesive strength of the material by
reinforcement action thereby reducing the degree of molecular diffusion across the interface of
butyl rubber. However, the average depth of penetration of the inter-diffused chains was still
adequate to form entanglement on either side of the interface, and thus offered greater resistance
to peeling, resulting in high tack strength measurements. It must be specified here that,
improvement in tack strength of the butyl rubber was only achieved at critical nanoclay loading
above 8phr, in proportional to that of the neat rubber. For example, the tack strength of the
sample loaded with 16phr was 35% higher than the tack strength of the neat butyl rubber.
Additionally, contact angle measured was done to examine the surface properties of the
specimen. The finding obtained presented no any significant interfacial property change, as the
sample’s surfaces still remained rubbery for interfacial diffusion and bond formation to occur.
Keywords : nanocomposites; autohesion; inter-diffusion; brominated isobutene isoprene rubber
I. Introduction
Autohesion is the resistance to separation of two uncured bonded identical elastomer that
have been joined together for a period of time under a given temperature and pressure.1-5 When
two rubber surfaces are joined above their glass transition temperature, distinct macroscopic
interface between the joining pieces slowly disappears with time, and the mechanical strength of
the interface progressively increases.
Generally, plasticizers like oil facilitate molecular diffusion across the interface by
diminishing the entanglement density of the rubber chains, but diluted, interdiffused rubber
chains are more easily separated than the neat sample, and hence the tack strength reduces.4-6 On
the other hand, low molecular weight tackifying resins are added to synthetic elastomer
compounds to enhance tack and to prevent tack decay.4,6-8 Similar to the action of oil, tackifying
resins also reduce the entanglement density of the base elastomer, however, the interdiffused
chains diluted with tackifiers resist separation significantly more than those diluted with oil.
It has also been reported that the addition of reinforcing fillers like carbon black can
influence the tack strength of elastomers.9-13 Nanoclays have been used as a potential reinforcing
agent for various elastomers over the past decade.14-16 Such nanoclays offer a wide array of
property improvements at very low filler loadings, owing to the dispersion of few nanometers
thick clay platelets of high aspect ratio. For example, researchers have extensively studied the
effect of different platelet-like montmorillonite (MMT) nanoclays and needle-like sepiolite
nanoclays on the physico-mechanical properties of various elastomers.16-22
In this study, the effect of nanoclay (Cloisite 20A) on the autohesive tack behavior of
uncured brominated isobutyl isoprene rubber (BIIR) has been investigated with a special regard
to (a) nanoclay concentration and (b) morphology of rubber-clay nanocomposites. Bond
formation (self-diffusion) and breaking ability (strength of the interface) has been analyzed by
studying various distinct tack governing parameters like green strength, 180o peel adhesion test
and the tack strength of the nanocomposites. The BIIR was selected due to its appreciable
viscoelastic behavior and its wide application in the tire industry as inner liner tubes. Other tests
such as stress-strain, X-ray diffraction, transmission electron microscope, thermogravimetric
analysis were also carried on the nanocomposites to fully appreciate the dispersion of the
nanoclays in relation to the tackification behavior of the filled and unfilled compounds.
II. Experimental
Brominated isobutylene isoprene rubber (BIIR) was supplied by Hancook Tire Co., Korea.
organo-modified clay (Cloisite 20A, modified by dimethyl dehydrogenated tallow and
quaternary ammonium chloride) was supplied by the Southern Clay Products Inc.
Neat rubber (BIIR) was mixed in an internal mixer (B-type Banbury mixer, S. Korea) at 150°C,
60 rpm rotor speed for 3 minutes followed by the addition of Cloisite 20A clay for an additional
mixing time of 5 minutes. The mixtures were sheeted out from a two-roll mill (Farrel 8422,
USA). A rectangular sheet of samples (15cm x15cm x 2mm) were molded by electrical hot press
machine (Caver, WMV50H, USA), at a pressure of 1.1GPa for 5 minutes and at 100°C
temperature. Samples were pressed in-between a Mylar film sheet of ≥1mm thickness fabric.
They were then left for overnight and strips of 15cmx1.5cmx 2mm sizes were cut out for the peel
test. Samples were also prepared in the same procedure and standard dumbbell shapes were cut
out for the green strength test. However, in this case pressing of the sample was done in-between
two aluminium foils. Table 1 below depicts the sample formulation and designations.
The tensile strength measurement was carried out according to ASTM D412 standard by
using (LLOYD instrument, UK). Prior to the test, the aluminum foils covering the faces of the
dumbbell-shaped specimens were removed and then subjected to stress-strain test at a cross-head
speed of 500 mm/min and 25oC temperature. The maximum tensile stress was taken as the green
strength from the generated stress-strain curve and at least four samples were tested for each
composition and averaged.
The tack strengths of the samples were also examined. In this case, the Mylar films were
removed from the surface of the two strips, and two surfaces were gently brought together. A
pressure system of about 0.13MPa was applied gently and uniformly on each side of the bonded
strips with an area of surface contact of 1.5cm2, at equal rate. At least each press lasted for a
contact time of about 5s (10s in all for the two sides). The slightly given pressure at the short
chosen time was to ensure inter-diffusion and the removal of entrapped air bubbles at the
interface of the bonded strips. Immediately after the contact time was reached upon pressing,
sample was removed immediately and subjected to the peel test using the tensile tester. The
peeling test was based on 180 orientation (T-type peeling test) and at a speed of 100mm/min.
The testing machine generates a plots of the force (N) required to separate the bonded strips
against the distance of separation (mm). A typical illustration of sample under 180 orientation
peeling at a peeling speed of 100mm/min is shown in Figure 2. The average forces required for
separating bonded strips were recorded and together with the samples width, the tack strength Ga
(N/m) was calculated using the equation below:
Ga=2 Fw (1)
where F is the estimated average peel force (N) and w is the width (m) of the strips of the
samples. For each composition, four samples were tested, and averaged.
III. Results and Discussion
The X-ray diffractograms of the pristine Cloisite 20A nanoclay and BIIR-Clay
nanocomposites are shown in Figure 3. From the X-ray diffractogram of the pristine clay the
basal characteristic peak (001) for Cloisite 20A appears to be at 2θ = 7.2° (d001=1.23 nm). For the
binary nanocomposites of BIIR-cloisite clay, this peak shifted to lower angles around 2θ = 5.5°
suggesting an increase of the d-spacing of the galleries of the organoclay. The d-spacing of the
nanocomposite was higher than the virgin organoclay. Moreover these peaks were broad and this
signifies an intense intercalation of polymer chains inside the clay layers (see Figure 3).
The SEM micrographs of fractured uncured neat rubber B and a representative of an
uncured binary nanocomposite are shown in Figure 4 (a and b). From the SEM micrograph, it
can be seen that the neat BIIR phase is smoother on the surface and regular in shape with less
number of troughs as compared with the nanocomposite. Yet, some tiny whitish spots which
could be an evidence of the presence of nanoclays were observed on the surfaces of the highly
filled nanocomposite (see Figure 4b). Since signs of agglomerates present were unusable with
this kind of test, as such TEM analysis was adopted to confirm such tendencies.
In order to be certain of successful nanoclay dispersion within the bulk butyl rubber,
TEM analysis was carried on the samples and the results (micrograph) is as shown in Figure 5. In
the case of BIIR-clay nanocomposites, a coarse morphology with tactoids of various sizes is
observed which corresponds to intercalation of polymer chains within the clay galleries. This
agrees well with the WAXD results. Even though, the TEM micrographs of the filled samples
confirmed successful dispersion of nanoclay within the rubber matrix, an evidence of bulk clay
agglomerates was observed(indicated as dark spot in the TEM images). It must be noted that
these agglomerated regions were common among the specimens with higher nanoclay loading. A
typical example is as shown in Figure5b.
IV. Conclusions
The effect of organoclay (Cloisite 20A) on the autohesive tack (self-adhesion) behavior of
brominated isoprene isobutylene (BIIR) rubber has been investigated. Even though,
morphological study by SEM did not show any remarkable features of the nanoclay scattering,
yet successful dispersion of the nanoclays was confirmed by the TEM and XRD analysis. For
example the XRD spectrum revealed an intense intercalation of the polymer chains within the
interlayer spacing of the nanoclays. Despite this, evidence of some agglomerated regions was
observed and this was common among the samples with the high clay concentration.
Acknowledgements
The support (1215-2-3-0327) of the MEST (Ministry of Education Science and
Technology) and KOFST (The Korean Federation of Science and Technology) is duly
acknowledged.
References
1. A. Aradian, E. Raphael, and P.-G. de Gennes, “Strengthening of a polymer interface:
interdiffusion and crosslinking”, Macromolecules, 33, 9444 (2000).
2. Y. H. Kim and R. P. Wool, “A theory of healing at a polymer-polymer interface”,
Macromolecules, 16, 1115 (1983).
3. S. S. Voyutskii, “Autohesion and adhesion of high polymers”, Interscience Publishers,
New York, 1963, Chapters 1 and 2, pp. 5-59.
4. G. R. Hamed, “Tack and green strength of elastomeric materials”, Rubber Chem. Technol.,
54,
576 (1981).
5. C. K. Rhee and J. C. Andries, “Factors which influence autohesion of elastomers”, Rubber
Chem. Technol., 54, 101 (1981).
6. G. R. Hamed and G. D. Roberts, “Comparison of the effect of a hydrocarbon oil and a
terpene tackifier on the autohesion of styrene-butadiene rubber”, J. Adhes., 47, 95 (1994).
7. A. K. Bhowmick, P. P. De, and A.K. Bhattacharyya, “Adhesive tack and green strength of
EPDM rubber”, Polym. Eng. Sci., 27, 1195 (1987).
8. K. D. Kumar, A. H. Tsou, and A. K. Bhowmick, “Unique behavior of hydrocarbon resin
tackifier on unaged and aged Tack of Brominated Isobutylene-co-p-methylstyrene (BIMS)
rubber”, J. Adhes. Sci. Technol., 22, 2039 (2008).
9. J. R. Beatty, “Tel-tak: A mechanical method for estimating both tackiness and stickiness of
rubber compounds”, Rubber Chem. Technol., 42, 1040 (1969).
10. W. F Busse, J. M Lambert, and R. B. Verdery, “Tackiness of GR‐S and other elastomers”,
J. Appl. Phys., 17, 376 (1946).
11. R. K. Beckwith, L. M. Welch, J. F. Nelson, A. L. Chaney, and E. A. McCracken, “Tack of
butyl and natural rubbers”, Rubber Chem. Technol., 23, 933 (1950).
List of Tables
Table 1. Formulation (phr).
Table 2. Tensile Properties of the Filled and Unfilled Butyl Rubber at 25oC Testing
List of Figures
Figure 1. Autohesion mechanism in an elastomer.
Figure 2. A photo of autohesive tack measurement by 180o peel test geometry.
Figure 3. X-ray diffractograms of the pristine Cloisite 20A clay and BIIR-Cloisite 20A
nanocomposites B, BC4 and BC16.
Table 2. Tensile Properties of the Filled and Unfilled Butyl Rubber at 25oC Testing
Specimen B BC4 BC8 BC16
Tensile Strength(KPa) 304 314 327 389
Modulus (KPa) 134 151 157 219
Elongation at break (%) ≥2000 ≥2000 ≥2000 ≥2000
Figure 2. A photo of autohesive tack measurement by 180o peel test geometry.
20
Figure 3. X-ray diffractograms of the pristine Cloisite 20A clay and BIIR-Cloisite 20A
nanocomposites B, BC4 and BC16.